Fission vs. Fusion – What’s the Difference?

Editor's note: This article was originally published on January 30, 2013. It has been revised, updated and republished. 

Inside the sun, fusion reactions take place at very high temperatures and enormous gravitational pressures

Look up during the day to see one of the most powerful examples of a nuclear reactor: the sun. Inside the sun, fusion reactions take place at very high temperatures and enormous gravitational pressures.

The foundation of nuclear energy is harnessing the power of atoms by splitting apart, a process called fission, or combining them, called fusion. Both fission and fusion alter atoms to create energy, but what is the difference between the two?

Fission, a term coined by scientists LIse Meitner and Otto Frisch, is named after the term “binary fission” in biology to describe cell division. Just as cell’s divide, in fission an atom splits into smaller particles. Fission takes place when a large, somewhatunstable isotope (atoms with the same number of protons but different number of neutrons) is bombarded by high-speed particles, usually neutrons. These neutrons are accelerated and then slammed into the unstable isotope, causing it to fission, or break into smaller particles. During the process, a neutron is accelerated and strikes the target nucleus, which in the majority of nuclear power reactors today is Uranium-235. This splits the target nucleus and breaks it down into two smaller isotopes (the fission products), three high-speed neutrons, and a large amount of energy. This resulting energy is then used to heat water in nuclear reactors and ultimately produces electricity. The high-speed neutrons that are ejected become projectiles that initiate other fission reactions, or chain reactions.

Nuclear Fission

Conversely, fusion takes place when two low-mass isotopes, typically isotopes of hydrogen, unite under conditions of extreme pressure and temperature. Atoms of Tritium and Deuterium (isotopes of hydrogen, Hydrogen-3 and Hydrogen-2, respectively) unite under extreme pressure and temperature to produce a neutron and a helium isotope. Along with this, an enormous amount of energy is released, which is several times the amount produced from fission.

Nuclear Fusion

While fission is used in nuclear power reactors since it can be controlled, fusion is not yet utilized to produce power. Some scientists believe there are opportunities to do so. Fusion offers an appealing opportunity, since fusion creates less radioactive material than fission and has a nearly unlimited fuel supply. These benefits are countered by the difficulty in harnessing fusion. Fusion reactions are not easily controlled, and it is expensive to create the needed conditions for a fusion reaction. However, research continues into ways to better harness the power of fusion, but research is in experimental stages, as scientists continue to work on controlling nuclear fusion in an effort to make a fusion reactor to produce electricity.

Both fission and fusion are nuclear reactions that produce energy, but the processes are very different. Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine together releasing vast amounts of energy. While different, the two processes have an important role in the past, present and future of energy creation.

Comments (27)

Posted January 02, 2024 by Ron
Theodore Silver that greatly confuses me too. I wish someone could explain it to us.
Posted December 01, 2023 by Bob the bizon
Just here cause today dec 1 2023 japan has launched first Fusion reactor..(experimental)
Posted November 13, 2023 by Leo Savas
The drive toward higher energy demand is driven by unlimited and uncontrolled materialistic dispositions which in the majority of all cases/reasons it is irrational and of little benefit to the advancement of our humanity. In this condition humanity justifies energy producing methods which are not necessarily safe from a military perspective or from a climatic perspective. The most abundant safe energy source on our planet is water, wind, the earths magnetic field, the earths lava heat, and the energy from our sun. And all the intellectual capability of humanity should be directed at finding a more efficient way of utilizing these abundant resources towards energy creation and distribution. The nuclear technology has simply being a convenient energy source driven by military objectives .for the people who are driven by the drive toward dominance, power prestige and authority over others. So lets move on from these primitive human dispositions by inventing safer technologies for our planet and our humanity and lets at the same time remove such disposed human personalities from leadership roles.
Posted October 13, 2023 by Steven Johnson
Reply to George Bombadeer in November 2021, great idea. How big a structure do you think would be appropriate. I believe it is obvious that it would need to be built in orbit. I believe a 500 mile diameter would be about right
Posted July 23, 2023 by Dan De
Forget everything you have been told about Atomic Energy. For years the public has been influenced by people who have their own agenda and we have been influenced by Coal, Oil, and Gas companies. We have been duped into thinking cheaper oil and gas prices are the answer to our energy needs. Nuclear technology is safe effective and by far the cleanest most efficient and most non-polluting answer for us to reach our goal of Zero Carbon Emissions by 2050. Record heat waves and wild weather happen yes they do! We have seen this year the most drastic climate change events. wildfires in Canada sweltering heat in the southwest, record high temperatures, Energy demands skyrocketing with more and more coal and oil to meet these demands. Even with all of the wind farms and solar panels they will not meet the objective of reducing the CO2 gases and pollution of the atmosphere, additionally, toxic waste being created by coal burning is a real problem for disposal ashes and acid are the problem these pollute groundwater and dumping these in the ocean is not the answer neither is burning more coal or oil to dig pits for disposal. Please take a look at a new documentary that is both informative and eye-opening. Nuclear Now 2022 by Director: Oliver Stone The Future of our planet hangs in the balance, if we just sit back and let the lobbyists control the future we will have sentenced future generations to a wild and hostile world. Floods, Famine, and worse will happen if we don't act now! I have no personal goal in mind other than to present fact not fiction . Decide for yourself and tell your senator or congressman or woman, " Lets cut the red tape and America can be #1 in Innovation and Conservation" Let everyone get behind a nonpolluting energy solution,
Posted June 06, 2023 by drs
Fusion drives our Sun and is about 1,000 times more powerful than fission, which itself is about 1,000 more powerful than chemical. Fusion is so hot its hard to build containment vessel, not that fission is easy.
Posted February 09, 2023 by clay wyatt
Where is the author
Posted January 17, 2023 by John Stevens
The Urantia Book Paper 41 Physical Aspects of the Local Universe 7. Sources of Solar Energy 41:7.1 (463.1) The internal temperature of many of the suns, even your own, is much higher than is commonly believed. In the interior of a sun practically no whole atoms exist; they are all more or less shattered by the intensive X-ray bombardment which is indigenous to such high temperatures. Regardless of what material elements may appear in the outer layers of a sun, those in the interior are rendered very similar by the dissociative action of the disruptive X rays. X ray is the great leveler of atomic existence. 41:7.2 (463.2) The surface temperature of your sun is almost 6,000 degrees, but it rapidly increases as the interior is penetrated until it attains the unbelievable height of about 35,000,000 degrees in the central regions. (All of these temperatures refer to your Fahrenheit scale.) 41:7.3 (463.3) All of these phenomena are indicative of enormous energy expenditure, and the sources of solar energy, named in the order of their importance, are:
Posted January 12, 2023 by Gabriel Bisbee
I'm pretty sure that the reason one would get massive amounts of energy from both reactions is because fusion starts with unstable isotopes of hydrogen and ends with stabler isotopes of helium, and fission starts with unstable isotopes of uranium and ends with stabler isotopes of other elements. In other words, fusion's reactants are different from fission's products, and vice versa. It takes less energy to break apart uranium into other elements than to form uranium from other elements, and it takes less energy to form helium from hydrogen than to break helium into hydrogen, if that makes more sense. I may be wrong, though.
Posted December 12, 2022 by Kevin Crean
Is it fission or fusion that is the "power of 1000 suns"? I have a childhood recollection from the NY World's Fair that the GE pavilion said it was fission (1964), but I've been contradicted often. Is it one or the other? Was it different then than now? Thank you. PS back then, my father told me only nuclear power can serve the needs of a society requiring 'instant-on' televisions and hair dryers, toasters, etc.
Posted May 09, 2022 by Ellen Morton
Nuclear thermal propulsion systems are powered by nuclear fission. Learn more in this article by the United States Department of Energy:
Posted May 09, 2022 by Ellen Morton
Nuclear thermal propulsion systems are powered by nuclear fission. Learn more in this article by the United States Department of Energy:
Posted May 05, 2022 by Subham Maharana
What iit is used in the space shuttle during the leaving that area ????????????
Posted March 30, 2022 by Hugh Mongous
According to all known laws of aviation, there is no way a bee should be able to fly. Its wings are too small to get its fat little body off the ground. The bee, of course, flies anyway because bees don't care what humans think is impossible.
Posted March 03, 2022 by Addy Neal
So fusion and fission which one is energy intensive and why
Posted February 23, 2022 by Dan Guiser
I went to a seminar, presented by an energy company, and the presenter talked about the difference between fission and fusson reactors for energy production. He said that presently, fusson reactors were not yet available. That was in 1973. Why is it taking so long?
Posted January 13, 2022 by Caleb Kunz
Fusion: "The power of the Sun, in the palm of my hand." - Dr. Otto Octavius/Doctor Octopus/Doc Ock (Spider-Man 2, 2004)
Posted December 29, 2021 by Nikita kanthe
Posted December 25, 2021 by Rick
To answer Tony Wallace's question, I believe the energy released in fusion comes from the fact that the binding energy required to hold the initial hydrogen isotopes together is less than the binding energy required to hold the resulting Helium atom together. In fission the reverse is true. The binding energy required to hold the U-235 atom together is less than the binding energy of the resulting new atoms. By "binding energy" I'm referring to the strong force caused energy required to keep the protons from flying apart in the initial atoms.
Posted November 15, 2021 by Edward Brady
SMR - Small modular Reactors
Posted November 09, 2021 by George Bombardier
Aren't we still just boiling water to drive turbines? We need to figure out a way to control fusion in a continuous bust similar to a white star only in miniature size, then surround it with solar panels to convert that power to electricity.
Posted October 21, 2021 by Tony Wallace
As an old thermodynamics engineer I have always subscribed to the energy balance principle that you can never get something for nothing. Yet her we appear to have two opposite processes; one which combines two light particles to form a heavier particle, and the other which splits a heavy particle into two lighter particles; and both give off vast amounts of energy. This appears to me to be the perfect perpetual motion situation; which I have always believed was impossible to achieve. So, my question is where did all the energy that these two processes are releasing come from in the first place?
Posted October 19, 2021 by Theodore Silver
It is my (rudimentary) understanding, as a general proposition, that when one or more relatively unstable species are subjected to such processes as one or more relatively stabler species, energy is released (the process is "exothermic.") Concomitantly if one or more relatively stabler species are subjected to such processes as produce one or more relatively unstable species, energy is consumed (the process is "endothermic.") Mindful of that proposition (if accurate), I am at a loss to understand how it is that (1) disrupting an intact nucleus (fission) releases energy (exothermia), and (2) creating new nuclei (fusion) also releases energy (exothermia). In my ignorance, I'm not understanding how to processes each (seeming to be) the converse of the other -- both release energy. Can you help me?
Posted October 05, 2021 by Charles
These Mathematics are thoroughly confusing, but it looks like you know what you're saying so ok. What happens if fusion astronomy alludes to the fascinating work of herpetology which changes my hypothesis of the art of breaking down an atom?
Posted September 16, 2021 by learn
YOU spelled wrong LIse Meitner. You wrote lisa
Posted September 09, 2021 by Andy Sefton
Fascinating - so well explained.
Posted August 30, 2021 by michael lombardi
i like it. this is very interesting to me.

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